Automatic pressure reducing valves are used in water distribution systems to reduce pressure to a pre-determined value or set-point that is adequate but does not expose normal components, such as household hot water tanks, to overpressure. The set-point is a pressure downstream from the valve and is determined according to the use of the valve. Water distribution systems experience significant losses of drinking water due to leaks, and as volume lost through leakage is directly related to pressure in the system, the set-point is determined to provide sufficient pressure that meets criteria of the water utility, particularly under high or “peak” demand conditions that may occur when fire is being fought. The pressure required for peak demand is typically significantly higher than that required for “off-peak” or typical night time conditions.
Valves are known for which the set-point may vary, and thus the downstream pressure from the valve may vary. Such valves are utilized to maintain the set-point, or downstream pressure, relatively low for off-peak or night time operation, thus reducing losses in the piping, and to raise the set-point during high flow demand for daily peak or special conditions such as may occur during fire fighting. Two types of automatic flow control valves with variable set-points include electronically controlled valves and “flow-driven” valves. The electronically controlled valves are relatively complex and require an electrical power source, such as batteries or access to utility power lines. The electrical power source increases installation costs and servicing costs and, as a result, such valves are used in limited numbers. Flow-driven valves use fluid pressures to control actuation of the main valve, and thus are independent of external power sources and can be used in most locations.
One type of such valve is supplied by Bermad, a corporation of Evron, Israel. This valve has a flow sensor responsive to differential pressures which are applied on opposite sides of an actuator diaphragm. A rigid push rod transfers diaphragm movement to a controlling valve member of a pilot valve which in turn determines control pressure which actuates the main valve. The actuator diaphragm is immersed in fluid which severely limits movement thereof, and this restriction limits movement of the push rod and valve member thus reducing valve sensitivity. When the flow sensor of such a valve is subjected to unusually high flow, however, the main valve can be opened excessively, resulting in excessively high downstream pressures that may cause damage to equipment and household appliances that are exposed to such pressure.